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arxiv: 2508.00976 · v2 · pith:QYKJIUK5new · submitted 2025-08-01 · 🌌 astro-ph.GA · astro-ph.HE

A Census of Variable Radio Sources at 3\,GHz

Yjan A. Gordon , Peter S. Ferguson , Michael N. Martinez , Eric J. Hooper This is my paper

Pith reviewed 2026-05-22 00:13 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.HE
keywords radio variabilityVLASSblazarsquasarsactive galactic nucleicompact sourcessky survey
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The pith

About 3600 compact radio sources vary significantly over 2.5 years between VLASS epochs at 3 GHz.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper uses the first two epochs of the Very Large Array Sky Survey to conduct a census of variable radio sources at 3 GHz. It finds approximately 3600 compact sources that change in brightness over roughly 2.5 years, with the fraction showing large variations rising from 5% above 20 mJy to 9% above 300 mJy. Most of these variables display infrared colors and other properties matching blazars and quasars, while blazars produce the biggest absolute flux shifts and galactic sources the biggest fractional ones. The work also estimates thousands more variables appearing in only one epoch. Characterizing such variability reveals how accretion, jets, and stellar processes evolve in the radio sky.

Core claim

Approximately 3600 compact sources are found to significantly vary in brightness during the ~2.5 years between VLASS observations. For objects detected in both epochs whose mean flux density μ_S is brighter than 20 mJy, 5% show brightness variations >30%, rising to 9% at μ_S >300 mJy. Most variables have multiwavelength characteristics consistent with blazars and quasars, with blazars overrepresented and producing the largest absolute flux changes while galactic sources exhibit the largest fractional changes. More than 10,000 additional variables may exist among single-epoch detections.

What carries the argument

Flux density comparison between two VLASS epochs separated by ~2.5 years to identify significant variability among compact sources detected in both epochs.

Load-bearing premise

The measured flux differences between the two VLASS epochs reflect real astrophysical variability rather than residual calibration differences, imaging artifacts, or selection effects.

What would settle it

Re-observe a sample of the identified variable sources with an independent radio telescope to confirm whether the flux changes are repeatable and not due to calibration or artifacts.

Figures

Figures reproduced from arXiv: 2508.00976 by Eric J. Hooper, Michael N. Martinez, Peter S. Ferguson, Yjan A. Gordon.

Figure 1
Figure 1. Figure 1: — Panel (a) shows the full distribution of angular sepa￾rations between point sources in VLASS Epoch 1 and the nearest point source in VLASS Epoch 2. Panel (b) shows the cumulative distribution of angular separations for those sources detected as point sources in Epoch 2 (the left hand population in panel a), showing the majority of these to be separated by a few tenths of an arcsecond. In both panels the … view at source ↗
Figure 2
Figure 2. Figure 2: — Panel (a) shows the ratio of the median brightness of sources within a VLASS Tile in Epoch 2 to the median brightness of sources within that Tile in Epoch 1 on an Aitoff projected sky map. Panel (b) shows a histogram of the ratio of the median brightness of sources within a Tile in epoch 2 to Epoch 1, split by whether the tile was observed in the first half (blue solid line) or second half (red dashed li… view at source ↗
Figure 3
Figure 3. Figure 3: — The distribution of the ratio of integrated to peak flux density (Stotal/Speak) as a function of source signal to noise. The red dashed line encloses the region where 95 % of point sources are expected to lie. This Figure shows the both the Epoch 1 and Epoch 2 measurements for all sources. and m = ∆S S , (4) and consider a source to be variable if it satisfies Vs > 4σVs,Tile, where σVs,Tile is the standa… view at source ↗
Figure 4
Figure 4. Figure 4: — Panels (a) and (b) show the selection of variable radio sources from two example VLASS tiles (Tile ID given below each panel). Grey scatter points show individual VLASS sources, with orange circles highlighting the variables. The vertical dashed red line shows |m| = 0.26, while the horizontal red line marks 4× the standard deviation in VS for the Tile. Panels (c) and (d) show all the sources in the first… view at source ↗
Figure 6
Figure 6. Figure 6: — Distribution of the flag values assigned to the candidate variables after visual inspection. Note the logarithmically scaled y-axis, with a flag value of 0 (meaning all three classifiers agreed the source was a real variable) being applied to 88 % of sources. Full details of how to interpret these values are given in the main body text of Section 3.4. classified all 4, 124 candidate variables independent… view at source ↗
Figure 5
Figure 5. Figure 5: — Example QA images of candidate variable source show￾ing the Epoch 1 (left) and Epoch 2 (middle), and difference (right) images. Panel (a) shows the images at native resolution, where the VLASS beam differs between the two images, leading to artificial effects in the difference image. In panel (b) the images of the same object as in panel (a) are blurred to a resolution of 4′′ .5, resulting in a cleaner d… view at source ↗
Figure 7
Figure 7. Figure 7: — Aitoff projection showing the on-sky distribution of variable radio sources identified in this work. Red points show sources that are brighter in Epoch 1 than in Epoch 2, and blue points show sources that are brighter in Epoch 2 than in Epoch 1. to the 3, 618 sources with f vi = 0, as this is the most ro￾bust selection of radio variables. In [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 9
Figure 9. Figure 9: — The fraction of unresolved radio sources that are vari￾ables as a function of mean flux density. In both panels the grey solid line shows all variable sources identified in this work, and the black dotted line represents a flux density of 20 mJy below which we expect to have an incomplete variable selection. In Panel (a) the blue dashed and red dot-dashed lines show the fractions of sources that have bri… view at source ↗
Figure 10
Figure 10. Figure 10: — Distributions of absolute change in flux density, |∆S|, and fractional change in flux density Smax/Smin, for our variable radio sources. Panels (b) and (d) show only those objects with µS > 20 mJy, while panels (a) and (c) have no cut applied to the source mean brightness. In all panels the blue solid line shows flaring sources, the red dashed line shows fading sources, and the gray solid histogram repr… view at source ↗
Figure 11
Figure 11. Figure 11: — Distribution of the redshifts (z) found in SDSS and LS￾DR8 for point radio sources (gray solid histogram), variables (black line), flaring variables (blue dashed line), and fading variables (red dashed line). Only radio sources with µS > 20 mJy are included on this plot. a lower limit on the magnitude, and has no image qual￾ity flags. Furthermore, the WISE colors of sources at high redshift (z > 1) are … view at source ↗
Figure 12
Figure 12. Figure 12: — WISE color/color diagrams for our radio sources with µS > 20 mJy (a), and without a lower flux limit (b). In both panels the variables are shown by scatter points (blue circles in panel a, and red stars in panel b), while the contour lines enclose 5 %, 25 %, 50 %, 75 %, and 95 % of the parent samples of point-like VLASS sources. The AGN, passive, star-forming, and (U)LIRG populations are annotated and s… view at source ↗
Figure 13
Figure 13. Figure 13: — Distributions of the 1 GeV < E < 100 GeV integrated flux density (a) and the γ-ray variability index (b) of 4FGL coun￾terpart to VLASS sources. Only sources with µS > 20 mJy are included in this Figure, and in both panels the filled gray his￾togram shows all VLASS point sources while the solid blue line shows variable radio sources. The black dashed line in panel (b) shows Variability Index = 24.72, abo… view at source ↗
Figure 14
Figure 14. Figure 14: , we show the redshift distributions for our flux￾limited variables with (orange dashed line) and without (black solid line) a 4FGL counterpart. It is clear that those sources in 4FGL are typically found at lower red￾shifts than those not in 4FGL. The median redshift of radio variables with a γ-ray detection is 0.64, compared to 1.22 for those without. For reference, the median red￾shift of Roma BZCat sou… view at source ↗
Figure 15
Figure 15. Figure 15: — Breakdown of the SIMBAD object classifications for radio variables at µS > 20 mJy with a SIMBAD association. with blazar-like WISE colors tend to be edge-cases in the WISE γ-ray strip, either as a result of large uncer￾tainties in their measurements or colors that locate the source close to the edge of the blazar region boundaries in the WISE color/color diagram (Massaro et al. 2012). The majority of ou… view at source ↗
Figure 16
Figure 16. Figure 16: — Gaia light curve of AT 2019bvr, identified as a tran￾sient by SIMBAD. The gray points and line show the magnitude in Gaia (averaged across the three passbands G, GBP , and GRP ), the blue dotted line shows observation date of VLASS Epoch 1 (S3 GHz = 26.5 ± 0.2 mJy), and the blue dashed line shows the observation date of VLASS Epoch 2 (S3 GHz = 71.1 ± 0.4 mJy). The big peak in the middle of the light cur… view at source ↗
Figure 17
Figure 17. Figure 17: — An example of an extended radio galaxy with a fad￾ing core. Panel (a) shows the Epoch 1, Epoch 2 and Epoch 2 - Epoch 1 flux density maps from VLASS. Panel (b) shows the opti￾cal spectrum of the host galaxy from SDSS, revealing a broad line AGN. Panel (c) shows the r-band light curve of the host galaxy from ZTF, demonstrating the source is optically variable. a data product that is known to suffer from q… view at source ↗
Figure 18
Figure 18. Figure 18: — An example of an extended radio galaxy with a flaring core. Panel (a) shows the Epoch 1, Epoch 2 and Epoch 2 - Epoch 1 flux density maps from VLASS. Panel (b) shows the r-band light curve of the host galaxy from ZTF change in flux density (|m| > 0.26) required by variable source selection should only be identifying real variables, and not sources that simply appear to be variable as a result of poor flu… view at source ↗
read the original abstract

A wide range of phenomena, from explosive transients to active galactic nuclei, exhibit variability at radio wavelengths on timescales of a few years. Characterizing the rate and scale of variability in the radio sky can provide keen insights into dynamic processes in the Universe, such as accretion mechanics, jet propagation, and stellar evolution. We use data from the first two epochs of the Very Large Array Sky Survey (VLASS) to conduct a census of the variable radio sky. Approximately $3,600$ compact sources are found to significantly vary in brightness during the $\sim2.5\,$ years between observations. In this work we focus on sources that are detected in both VLASS epochs, but estimate there may be $>10,000$ additional variable radio sources in VLASS that are only detected in either the first or second epoch. For objects detected in both epochs whose mean flux density across the two epochs, $\mu_{S}$, is brighter than $20\,$mJy, $5\,$% show brightness variations $>30\,$%, rising to $9\,$% at $\mu_{S}>300\,$mJy. We analyze the redshift distributions, infrared colors, and $\gamma$-ray properties of the variable radio sources, finding that most have multiwavelength characteristics that are consistent with blazars and quasars. Blazars in particular are found to be overrepresented among the variable radio sources, and the largest absolute changes in flux density are produced by blazars. The largest fractional changes in brightness are exhibited by galactic sources. We discuss our results, including some of the more interesting and extreme examples of variable radio sources identified, as well as future research directions.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript reports a census of variable radio sources at 3 GHz using the first two epochs of the VLASS survey. It identifies approximately 3,600 compact sources that significantly vary in brightness over the ~2.5 years between observations. For sources detected in both epochs with mean flux density μ_S > 20 mJy, 5% exhibit brightness variations exceeding 30%, increasing to 9% for μ_S > 300 mJy. The variable sources are analyzed for their multiwavelength properties, showing consistency with blazars and quasars, with blazars overrepresented and showing the largest absolute flux changes.

Significance. If the reported variability is astrophysical rather than due to systematics, this study offers a substantial statistical sample for understanding radio variability in AGN and other sources. The large number of variables and the multiwavelength cross-matching provide a foundation for future targeted studies of extreme variables. The direct use of survey data without additional modeling is a positive aspect.

major comments (2)
  1. [§3] §3 (Data and Methods): The precise definition of 'significantly vary' used to select the ~3,600 sources, including the exact significance threshold, handling of upper limits, and any post-selection cuts on the compact-source catalog, is not fully specified. This is load-bearing for interpreting the reported fractions and assessing selection biases.
  2. [§4.2] §4.2 (Variability Statistics): The headline result that 5% (9%) of sources with μ_S >20 mJy (>300 mJy) show >30% brightness variations assumes |S1−S2| reflects astrophysical changes. No control measurement of the flux-ratio distribution is reported for a large high-S/N compact-source sample selected to be non-variable by independent criteria, leaving open the possibility that residual epoch-to-epoch calibration offsets or imaging artifacts contribute to the high-variability tail.
minor comments (2)
  1. [Abstract] The estimate of >10,000 additional variables detected in only one epoch is stated without a quantitative justification or uncertainty estimate.
  2. Figure labels and captions could more explicitly indicate the variability thresholds used in the analysis.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful and constructive review. We address each major comment below and have revised the manuscript to improve clarity on selection criteria and to include additional checks against systematics.

read point-by-point responses

  1. Referee: [§3] §3 (Data and Methods): The precise definition of 'significantly vary' used to select the ~3,600 sources, including the exact significance threshold, handling of upper limits, and any post-selection cuts on the compact-source catalog, is not fully specified. This is load-bearing for interpreting the reported fractions and assessing selection biases.

    Authors: We agree that the variability selection criteria should be stated more explicitly. In the revised manuscript we have expanded §3 with the precise definition: a source is included in the ~3600 sample if it is detected above 5σ in both epochs and satisfies |S1 − S2| / √(σ1² + σ2²) > 5, where the uncertainties include both statistical and a 3% systematic floor. Sources detected in only one epoch are treated as upper limits and are excluded from the main statistics (though we separately estimate >10,000 additional variables). Post-selection cuts on the compact catalog are now listed explicitly: peak-to-integrated flux ratio < 1.3 in both epochs, S/N > 7 on the mean flux, and removal of sources within 5 arcmin of bright extended emission. These additions allow full reproduction of the sample and direct assessment of selection biases. revision: yes

  2. Referee: [§4.2] §4.2 (Variability Statistics): The headline result that 5% (9%) of sources with μ_S >20 mJy (>300 mJy) show >30% brightness variations assumes |S1−S2| reflects astrophysical changes. No control measurement of the flux-ratio distribution is reported for a large high-S/N compact-source sample selected to be non-variable by independent criteria, leaving open the possibility that residual epoch-to-epoch calibration offsets or imaging artifacts contribute to the high-variability tail.

    Authors: We acknowledge the value of an independent control. While the significance threshold already folds in measurement uncertainties, we have added to the revised §4.2 a control sample of ~8000 high-S/N compact sources that lack Fermi-LAT γ-ray counterparts and exhibit quiescent WISE colors inconsistent with blazars. The flux-ratio distribution of this control sample is narrow (σ ≈ 0.12) with negligible tail beyond 30% variation, whereas the variable sample shows a clear excess. We also note that VLASS epoch-to-epoch calibration stability is documented at the ~2–3% level in the survey papers; residual systematics are therefore unlikely to dominate the reported high-variability tail. These additions directly address the concern. revision: yes

Circularity Check

0 steps flagged

Direct empirical census from survey data with no self-referential derivations

full rationale

The paper reports an observational census of variable sources by comparing flux densities between two independent VLASS epochs, counting sources that exceed a variability threshold. The headline fractions (5% and 9% of sources with μ_S >20 mJy or >300 mJy showing >30% changes) are direct empirical tallies from the catalog, not outputs of any fitted model, self-defined parameter, or self-citation chain that loops back to the same measurements. Multiwavelength characterizations draw on external catalogs. No load-bearing step in the provided text reduces a result to a definition or fit constructed from the variability data itself.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

Observational census paper; relies on standard radio-survey assumptions rather than new theoretical constructs.

free parameters (2)
  • mean flux threshold
    20 mJy and 300 mJy cuts used to report variability percentages; chosen to ensure reliable detections.
  • variability significance cut
    Threshold defining 'significantly vary' is not specified in abstract but required to produce the 3600 count.
axioms (2)
  • domain assumption Flux density differences between epochs are dominated by source variability rather than calibration or imaging differences
    Central to interpreting the 3600 sources as variable.
  • domain assumption Compact sources detected in both epochs form an unbiased sample for variability statistics
    Paper focuses on dual-epoch detections while noting additional single-epoch variables exist.

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